feat: add folder picker for automatic project file loading

- Add "Load Project Folder" button with folder selection (webkitdirectory)
- Automatically load all project files (JSON, CSV, SVG, CSS) from selected folder
- Eliminate need to manually upload each file individually
- Show clear errors if referenced files are missing from folder
- Update all documentation to explain folder picker usage

This solves the browser security limitation where uploading a single
project.json doesn't allow automatic access to other files in the same
directory. Users can now select an entire project folder and all files
load automatically in one click.

Changes:
- index.html: Add folder input with webkitdirectory attribute and UI
- engine.js: Add folderInput event handler to process all files from folder
- README.md: Document folder picker as primary loading method
- WINDOWS_USAGE.md: Add folder picker as recommended Option 1
- TROUBLESHOOTING.md: Add section explaining project files not auto-loading
- CLAUDE.md: Document folder picker architecture for future instances
- Makefile: Update DIST_README.md template to mention folder picker

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

index.html

@@ -239,6 +239,23 @@
   <div id="fileManager" style="margin-bottom:16px; padding:16px; background:#f8f9fa; border:1px solid #e9ecef; border-radius:8px;">
     <h3 style="margin:0 0 12px 0; font-size:14px; font-weight:600; color:#495057;">Project Files</h3>
 
+    <!-- Quick Load: Project Folder -->
+    <div style="margin-bottom:16px; padding:12px; background:#e7f3ff; border:1px solid #b3d9ff; border-radius:6px;">
+      <div style="display:flex; align-items:center; gap:12px;">
+        <label class="upload-btn" style="background:#0066cc; flex-shrink:0;">
+          <input type="file" id="folderInput" webkitdirectory directory multiple style="display:none;" />
+          📂 Load Project Folder
+        </label>
+        <span style="font-size:12px; color:#004080;">
+          Select an entire project folder to load all files automatically
+        </span>
+      </div>
+    </div>
+
+    <div style="margin:0 0 12px 0; text-align:center; color:#6c757d; font-size:12px;">
+      — or load files individually —
+    </div>
+
     <div class="file-grid" style="display:grid; grid-template-columns: repeat(auto-fit, minmax(280px, 1fr)); gap:12px; margin-bottom:16px;">
       <div class="file-item">
         <div class="file-header">